Firefox only (Windows/Mac/Linux): Firefox extension UrlbarExt adds 7 new buttons to the Awesome bar with a wide range of genuinely useful functionality to power up your browsing experience.

We've mentioned this extension before, but it has since updated with more functionality and upgraded from experimental to officially approved by the Mozilla Add-ons site (no longer requiring a login to install). The functionality of the default buttons include, from left to right:

Finding new and useful Firefox extensions is a passion of ours, but sometimes we run across…
Read more Read more

Copy the current site URL or custom formatted URL.

Shorten the URL using your favorite service or right-click for more options.

Search the current site using Google for the selected keyword. Right-click opens a dialog for adding keywords.

Go up one level, or directly to the root of the current site with a double-click. Right-click gives you a list of levels to choose from.

Tag and bookmark the current page from a menu of tags.

Navigate through sequential URLs. (if the URL ends in a number, it will take you to the next numbered URL)

Surf anonymously using online phproxy servers. Right-click lets you view the Google Cache along with other options.

Once installed, the settings panel lets you fully customize which icons are displayed by default or hidden behind the "Grippy", along with a ton of other options.

Clicking on the Shorten URL button will use your default service, but right-clicking gives you extra options to email or twitter the last shortened URL, or even switch to a different URL shortening service.

The TinyURL tab in the preferences also lets you switch between your favorite service or add your own, but especially useful in this panel is the checkbox to automatically copy the shortened URL to the clipboard.

Clicking the tags button in the awesome bar will bookmark the current URL with the chosen tag, saving you from manually typing the tag each time you want to bookmark something (a pet peeve of mine).

Back in the preferences, the tags that show up in the list can be customized to your preferred set of Firefox 3 tags, and you can even configure the style of the items to put your favorite tags in bold text, for instance.

This extension is now a must-have in my toolbox just for the tag bookmarking functionality alone. Are there any extensions that you just couldn't live without? Tell us in the comments.